1. Field of the Invention
The present invention relates to a thermosensitive recording medium useful as a block copy film for flexography, gravure printing, offset printing and screen process printing.
2. Discussion of Background
A thermosensitive recording medium which comprises a transparent support and a thermosensitive recording layer formed thereon can produce an image therein when a dye and a color developer contained in the thermosensitive recording layer undergo the coloring reaction by the application of heat thereto. This type of thermosensitive recording medium is used by directly printing images thereon, for example, as an image formation film for overhead projectors and a second original for diazo copying. In addition to the above, this type of recording medium can also be used as a block copy film for flexography, gravure printing, offset printing and screen process printing when a predetermined image is thermally recorded in the thermosensitive recording medium.
When the above-mentioned image bearing thermosensitive recording medium is used as a block copy film for textile printing or flexographic printing, sufficient contrast between a colored portion and a background portion in the ultraviolet region is remarkably significant. If the contrast is insufficient, it is necessary to extend the ultraviolet exposure time when a form plate is prepared by exposing the image bearing thermosensitive recording medium to ultraviolet lamp, or at the worst, the image bearing thermosensitive recording medium may be useless as a block copy film.
For instance, when a transparent thermosensitive recording medium is used as a block copy film, a form plate for use in the flexographic printing can be obtained by following the procedures as shown below, with reference to FIG. 1:
(1) A photo-setting resin plate 1 is set on a glass sheet of an exposure apparatus. PA1 (2) A transparent thermosensitive recording medium 2 which has been subjected to thermal printing in order to produce a predetermined colored image therein is closely attached to the photo-setting resin plate 1. PA1 (3) A cover film 3 is overlaid on the image bearing thermosensitive recording medium 1. PA1 (4) Air remaining under the cover film 3 is driven out in the direction of the arrow using a vacuum pump 4. PA1 (5) Air bubbles remaining between the photo-setting resin plate 1 and the thermosensitive recording medium 2 are expelled therefrom, for example, by rolling a cylindrical jig over the cover film 3. PA1 (6) The image bearing thermosensitive recording medium 2 is exposed to ultraviolet lamps 5 so as to set a portion of the photo-setting resin plate 1 corresponding to a background portion of the image bearing thermosensitive recording medium 2. PA1 (7) A switch for the vacuum pump 4 is turned off and the cover film 3 is removed, and then the thermosensitive recording medium 2 is peeled from the photo-setting resin plate 1. The resin plate 1 is subjected to cleaning, thereby obtaining a form plate.
As shown in FIG. 2, if there remains an air bubble 6 between the thermosensitive recording medium 2 and the photo-setting resin plate 1, the thermosensitive recording medium 2 cannot come in contact with the photo-setting resin plate 1. The result is that the edge sharpness of a convex portion formed on the form plate is decreased, so that the quality of the obtained form plate is considerably impaired.
In particular, when the thermosensitive recording sheet is large size, for example, A0 size, it is very difficult to remove the air bubbles generating between the photo-setting resin plate and the thermosensitive recording sheet from the end of the thermosensitive recording sheet.
When the contact surface of the thermosensitive recording medium 2 with the photo-setting resin plate 1 is properly roughened, it is easy to remove the air bubbles generating between the recording medium 2 and the photo-setting resin plate 1 at the aforementioned ultraviolet exposure step. Therefore, it is proposed to employ a matte finished thermosensitive recording sheet.
However, when the surface roughness of the thermosensitive recording medium is increased to facilitate the removal of the above-mentioned air bubbles in the course of the making of a form plate, the thermosensitive recording medium cannot be brought into close contact with a thermal head in the course of thermal printing. As a result, thermal transmission becomes poor and uneven, so that the thermal sensitivity of the recording medium is lowered, and therefore, the coloring density of the obtained image is uneven and many non-printed spots are formed in a solid image area.
As mentioned above, the conventional thermosensitive recording medium has the problem that thermal printing cannot be uniformly performed because the surface of the recording medium is roughened in order to efficiently remove the air bubbles in the course of the making of a form plate. To solve the above-mentioned problem, the thermal energy supplied to the recording medium by the thermal head is increased. However, in such a case, the surface of the thermosensitive recording medium is made flat after the application of a large quantity of heat and pressure thereto, whereby the air bubbles are easily generated between the recording medium and the photo-setting resin plate in the course of the making of a form plate.
In Japanese Laid-Open Patent Application 9-86046 there is proposed a thermosensitive recording medium comprising a thermosensitive recording layer and a top layer provided thereon. The above-mentioned recording medium is designed for an image recording sheet by directly recording images therein, not for a block copy film for printing. In this application, the relationship between the thickness of the top layer and the average particle diameter of the filler particles for use in the top layer is specified in order to prevent the thermosensitive recording medium from sticking to the thermal head when thermal printing is carried out.
Furthermore, a thermosensitive recording medium disclosed in Japanese Laid-Open Patent Application 10-166737 is intended as a block copy film. However, the surface roughness and the dynamic friction coefficient of the top layer of the recording medium specified in this application are those obtained before the recording medium is subjected to thermal printing.
In the above-mentioned conventional thermosensitive recording media, the top surface layer is made flat after the completion of image formation by the application of thermal energy and pressure to the recording medium using a thermal head.